Plastisols are used in the car industry for undersealing and for seam-closing and sealing. Depending on the application aim, further aggregates are added to the plastisols, which are dispersions of fine polymer particles in liquid plasticizers. Such aggregates are flow-improvers, stabilizers, adhesion promoters, fillers and water-absorbing substances. Adhesion promoters are added to the plastisols to produce long-life adhesion of the plastisols to steel or galvanized, tin-plated or electro-dip coated steel sheets. As adhesion promoters for PVC plastisols, basic compounds such as polyaminoamides (PAA) are used, for example. In contrast, as adhesion promoters for polymethacrylic plastisols, usually basic vinyl imidazoles are used, to which polyaminoamides are optionally added as additional adhesion promoter component. The other aggregates are likewise often basic compounds, e.g. calcium oxide, barium oxide and calcium carbonate. With the final coating, the plastisols are coated over at the same time.
Since electro-dip coats, particularly cathodic electro-dip coats are used for priming steel parts and sheets, it is observed that the coats applied to the plastisols yellow with time. This problem particularly affects light, pastel-coloured and white coats, because with these the discoloration is more easily noticed, particularly if they are applied without an intermediate coat (filler). Yellowing is especially serious with white coats. Yellowing does not in most cases appear immediately after stoving of the top coating, but only after some time. This is not a question of a yellowing of the plastisol itself, which could possibly appear through the top-coat layer, but of a gradual discoloration of the top coat itself.
To solve this problem, it was proposed in DE-A-38 21 926 to add as additives to the plastisols acid cation exchangers, whose effect is seen in that upon stoving they bind liberated amines. The addition of acid substances however has the disadvantage that they reduce or even inhibit the effect of basic adhesion promoters. Further, acid components react with other basic substances optionally present in the plastisol such as calcium oxide and chalk, so that carbon dioxide and water form and the plastigels formed from the plastisols become porous.